During 24-31 March, activity at Santa Ana decreased to low levels in comparison to the previous 4 months of moderate activity. During the report period, seismicity was at relatively low levels, steam plumes occasionally rose ~200 m above the volcano (or 8,400 ft a.s.l.), and the daily sulfur-dioxide flux was between 500 and 1,000 metric tons. The Alert Level remained at red, the highest level, within 5 km of the volcano's summit crater.

Researchers from Michigan Technological University (MTU) and Servicio Nacional de Estudios Territoriales (SNET) visited the crater of Santa Ana on 28 June and 5 July 2007 to measure crater lake and fumarole temperatures, and to carry out routine water sampling.

Crater lake. The crater lake appeared yellowish-green and had a maximum temperature of 57.5°C, measured by a thermocouple at the northern shore. The crater lake was observed to have shifted westward in position since the 1 October 2005 eruption, drowning the main pre-eruption fumarole field to the W and receding from its eastern border (figure 13). A subaqueous hot spring was observed in the center of the lake at the end of a peninsula of exposed sediments (figure 14). The hot spring exhibited episodic pulses of bubbling water about every 5 minutes.

Figure 13. The yellowish-green acid crater lake of Santa Ana volcano as seen when viewed on 28 June 2007 looking towards the N. Photo taken by Anna Colvin.

Figure 14. Hot spring emerging in the acid lake at Santa Ana as seen 5 July 2007. Episodic upwelling of whitish fluid radiated out from the base of the large rock in the center of the photo. View is towards the SW; note geologist for scale. Photo taken by Matt Patrick.

Fumaroles. Crater fumaroles were observed to the W and S of the crater lake, and weak fumaroles were also observed on the upper wall above the flat area and below the SW crater rim. The southern crater fumaroles and the upper fumaroles were measured by thermocouple and radiometer (Extech 42545) (figure 15). Fumaroles to the W were not measured due to limited accessibility.

Figure 15. At Santa Ana, the location of fumarole measurements and the hot spring shown in the previous figure. View is towards the SW. Photo mosaic taken 5 July 2007 by Matt Patrick.

The seven largest southern crater fumaroles were measured along an E-W transect. The lower fumaroles emitted mainly water vapor, though some sulfur crystals and a weak sulfurous smell were present. Lower fumaroles temperatures ranged from 92.0 to 95.2°C, and thermocouple and radiometer measurements agreed very well (to within 3%). The upper fumaroles were diffuse and relatively weak, occurring in loosely consolidated tephra. The upper fumaroles emitted mainly water vapor and lacked sulfur deposits or sulfurous smell. Upper fumaroles temperatures ranged from 70.0 to 79.0°C, and thermocouple and radiometer measurements agreed well (to within 6%).

Weekly Reports - Index

During 24-31 March, activity at Santa Ana decreased to low levels in comparison to the previous 4 months of moderate activity. During the report period, seismicity was at relatively low levels, steam plumes occasionally rose ~200 m above the volcano (or 8,400 ft a.s.l.), and the daily sulfur-dioxide flux was between 500 and 1,000 metric tons. The Alert Level remained at red, the highest level, within 5 km of the volcano's summit crater.

During 24 February to 6 March, seismicity at Santa Ana was relatively stable, and the sulfur-dioxide flux was lower than during previous weeks. The level of water in the lagoon within the crater decreased significantly. The Alert Level at Santa Ana remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 17-24 February, volcanic activity was at moderate levels at Santa Ana. Seismicity was relatively stable, and steam plumes rose to low levels above the volcano. The sulfur-dioxide flux was similar to measurements from previous weeks. The level of water in the lagoon within the crater decreased significantly. The Alert Level at Santa Ana remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 3-10 February, volcanic activity was at moderate levels at Santa Ana. Seismicity was relatively stable, and steam plumes rose to low levels above the volcano. The sulfur-dioxide flux averaged 1,200 metric tons per day. The Alert Level at Santa Ana remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 27 January- 3 February, volcanic activity was at moderate levels at Santa Ana. On the 2nd, there was an increase in seismicity at the volcano, possibly related to an earthquake on the coast of Guatemala. There was also an increase in the sulfur-dioxide flux, with an average of 2,000 metric tons measured daily. Steam plumes rose to low levels above the volcano. The Alert Level at Santa Ana remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Seismicity at Santa Ana during 14-20 January 2006 was at normal levels. Degassing continued, with sporadic gas-and-steam emissions that rose about 200 m before dispersing. Sulfur dioxide flux, measured 6 km SW of the volcano, ranged from 163 to 1,578 metric tons/day. The hazard status remained at Alert Red, the highest level, within a 5-km radius of the central crater.

During 6-13 January, volcanic activity was moderate at Santa Ana. Seismicity was a bit over normal levels with small earthquakes occurring, which were interpreted as being associated with gas pulses. Continuous low-level emissions of steam and gas originated from the lagoon and from fumaroles within the crater. The sulfur-dioxide flux ranged between 544 and 2,300 metric tons per day. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 30 December to 6 January, volcanic activity was moderate at Santa Ana. Seismicity was a bit over normal levels with small earthquakes occurring, which were interpreted as being associated with gas pulses. Continuous low-level emissions of steam and gas originated from the lagoon and from fumaroles within the crater. Gas rose 200-500 m above the crater (or 8,400-9,400 ft a.s.l.) and drifted SW. The sulfur-dioxide flux ranged between 180 and 1,476 metric tons per day. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Observations of Santa Ana's crater on 28 December revealed that there were continuous emissions of steam and gas from the lagoon and fumaroles located within the crater. Gas rose 200-500 m above the crater (or 8,400-9,400) and drifted SW. On 30 December, seismicity at Santa Ana was above background levels. Small earthquakes occurred, which were interpreted as being associated with gas pulses. Gas emissions rose to low levels. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 21-23 December, seismicity at Santa Ana was above background levels. Small earthquakes occurred, which were interpreted as being associated with gas pulses. Gas emissions rose to low levels. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

During 14-20 December, seismicity at Santa Ana was above background levels. Small earthquakes occurred, which were interpreted as being associated with gas pulses. Gas emissions rose to low levels. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's central crater.

Seismic activity began to increase at Santa Ana on 26 November. At that time, hundreds of metric tons of sulfur dioxide were emitted from the volcano each day, which was not as high as levels measured prior to the 1 October eruption. Gas emissions rose to ~300 m above the volcano and only slight changes were noted in the color of the lagoon in the interior of the crater. SNET stated that the high level of activity indicated that an eruption could occur in the following days. During 7-12 December, activity was still at high levels. The Alert Level remained at Red, the highest level, within a 5-km-radius around the volcano's central crater.

During 23-28 November, seismicity at Santa Ana was above background levels. Small earthquakes occurred that were interpreted as being associated with gas pulses. The amount of gas emitted was low. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's central crater.

During 16-21 November seismicity at Santa Ana was above background levels; a minor increase on 17 November was probably due to strong wind disturbing the seismic equipment. Small earthquakes occurred that were interpreted as being associated with the fracturing of rocks and gas pulses. The amount of gas emitted was low. The daily sulfur-dioxide flux averaged about 1,000 metric tons during 11-17 November. The Alert Level within a 5-km radius around the volcano's central crater remained at Red, the highest level.

During 9-14 November, seismicity at Santa Ana was above background levels and slightly increased on 13 November. Small earthquakes occurred that were interpreted as being associated with the fracturing of rocks and gas pulses. The amount of gas emitted was low. The sulfur-dioxide flux ranged between 100 and 1,200 metric tons daily. The Alert Level within a 5-km radius around the volcano's central crater remained at Red, the highest level.

During 2-7 November, seismicity and volcanic activity remained at relatively low levels at Santa Ana. Small earthquakes occurred that were associated with the fracturing of rocks and gas pulses. Sulfur-dioxide emissions were low, with 100 to 200 metric tons recorded daily. Gas emissions rose to low levels above the volcano. The Alert Level within a 5-km radius around the volcano's central crater remained at Red, the highest level.

During 30-31 October, seismicity increased at Santa Ana. Volcanic activity appeared to slightly increase starting on 28 October. Sulfur-dioxide emission rates during 28 and 29 October averaged 257 metric tons per day. The Alert Level within a 5-km radius around the volcano's central crater was at Red, the highest level.

During 19-23 October, seismicity was relatively stable and there were low-level gas emissions at Santa Ana. On 22 October, a lahar was reactivated in the Potrero Arriba area, NE of the volcano. During 22-25 October, there was an increase in the amount of tremor at the volcano. In addition, seismicity associated with gas emissions slightly increased. The Alert Level within a 5-km radius around the volcano's central crater was at Red, the highest level.

During 5-11 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana. Inclement weather during much of the report period prohibited ground and satellite observations and sulfur-dioxide measurements. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater. Around the 11th, sulfur-dioxide measurements were at 600-700 metric tons per day. SNET noted that eruptive activity could continue at the volcano and an eruption similar to, or smaller than, the October 1 eruption could occur in the future. The Alert Level within a 5-km radius around the volcano's central crater was at Red, the highest level.

SNET reported that a sudden eruption at Santa Ana (also called Ilamatepec) on 1 October around 0820 produced an ash-and-gas plume to a height of ~10 km above the volcano (or 40,600 ft a.s.l.). According to the Washington VAAC, ash was visible on satellite imagery at a height of ~14 km (46,000 ft) a.s.l. Ash fell in towns W of the volcano, including in Naranjos, Nahuizalco, Juayúa, Ahuachapán, and La Hachadura. Volcanic blocks up to a meter in diameter fell as far as 2 km S of the volcano's crater. Lahar deposits were seen SE of the volcano. The Alert Level within a 4-km radius around the volcano's central crater was raised to Red, the highest level. According to news reports, two people were killed by landslides (possibly caused by heavy rain in the area) in the town of Palo Campana, and thousands of residents near the volcano were evacuated. As many as 1,400 hectares of crops were damaged by ash.

Prior to the eruption, significant changes in seismicity were not noted. On 3 October, after the eruption, seismicity fluctuated and small explosions occasionally occurred. Earthquakes associated with explosions were recorded. In addition, there was a decrease in the amount of sulfur dioxide emitted from the volcano. SNET noted that eruptive activity could continue at the volcano.

During 21-26 September, seismicity and gas emissions were above normal levels at Santa Ana as they had been since 27 July. Microseismicity remained at relatively high levels. During the report period, gas plumes rose to a maximum height of ~1 km above the volcano (or 11,000 ft a.s.l.) on 26 September. During a visit to the crater on 21 September, observers noted that the summit crater lagoon had become greener and small rock slides occurred in a fumarolic area. Santa Ana remained at Alert Level Yellow Phase 1.

During 15-19 September, seismicity and gas emissions were above normal levels at Santa Ana as they had been since 27 July. Microseismicity remained at relatively high levels. During the report period, gas plumes rose to ~500 m above the volcano (or 9,400 ft a.s.l.) and the sulfur-dioxide flux reached a maximum of 3,320 metric tons per day on 16 September. No significant changes were seen at the volcano's crater when observed on 19 September in comparison to 13 September. Intense degassing continued and the lagoon remained a "dark coffee" color. Incandescence was visible inside some cracks. Santa Ana remained at Alert Level Yellow Phase 1.

During 7-12 September, seismicity and gas emissions were above normal levels at Santa Ana as they had been since 27 July. Microseismicity increased significantly on 12 September. During a visit to the volcano on 8 September, larger areas of incandescence were visible at a field of fumaroles than during a visit on 29 August. During the report period, gas plumes rose to ~500 m above the volcano (or 9,400 ft a.s.l.) and the sulfur-dioxide flux was over 1,000 metric tons per day. Satellite imagery showed a thermal anomaly at the volcano on several days. Santa Ana remained at Alert Level Yellow Phase 1.

During 1-6 September, seismicity and gas emissions were above normal levels at Santa Ana as they had been since 27 July. Tremor continued to be recorded, and on 2 August a cluster of at least eight small earthquakes occurred. The earthquakes were not felt by local residents. During the report period, gas plumes rose to ~500 m above the volcano (or 9,400 ft a.s.l.) and the sulfur-dioxide flux was over 1,000 metric tons per day. Satellite imagery from 5 September showed a thermal anomaly at the volcano. Santa Ana remained at Alert Level Yellow Phase 1.

SNET reported a significant increase in seismic activity at Santa Ana (also called Ilamatepec) on the night of 27 August. A cluster of 17 volcano-tectonic earthquakes were recorded, with four located S of the volcano. Afterwards, continuous high-frequency tremor was recorded until at least 30 August. Observations made on 29 August revealed incandescent rocks in the fumarole field. The incandescence was due to the hot gases emitted from the fumaroles heating the rocks. A significant increase in sulfur-dioxide emission was recorded, and gas-and-steam plumes rose 500-1,000 m above the volcano's crater (or 9,400-11,000 ft a.s.l.). As a safety measure, access to the volcano's crater was restricted to visitors.

Prior to the current increase in activity, strong degassing had been measured at the volcano since June 2004. An ash emission occurred on 16 June 2005, and a slight increase in seismicity and a significant increase in gas emission was measured from 27 July until at least 30 August.

Beginning on 12 January several news reports stated that increased volcanic activity occurred at Santa Ana volcano. The Washington VAAC reported that an eruption occurred at 1800 on 16 January that sent ash to ~3.7 km a.s.l. Local observations by volcanologists revealed that an eruption did not occur and no new lava or magma was in the summit crater. Glowing cracks that were visible at night were determined to be an existing fumarole field with measured temperatures of 550 °C. Scientists believe that a magnitude 7.7 earthquake that occurred off the coast of Central America at 1133 on 13 January, killing several hundred people, did not cause an increase in activity at the volcano. Since 12 January there have been reports of increased gas emissions and the volcano is being closely monitored for any changes in activity.

The acid lake within the summit crater of Santa Ana was extensively sampled by a scientific team using a small boat on 28-29 January 2000. The ambient lake temperature was ~18.9°C with 1.01 pH. A fumarole field on the crater wall adjacent to the lake had a maximum temperature of 523°C. A hot spring vent near the fumarole field feeds warm water (80°C) into the lake. This new lake sampling allows comparisons with data taken in 1992-93.

Santa Ana, El Salvador's highest volcano, is a large stratovolcano immediately W of Coatepeque Caldera. The broad volcano summit is cut by several crescent craters. A series of parasitic vents and cones have formed along a fissure system that extends from near the town of Chalchuapa N of the volcano to the San Marcelino cinder cone on the SE flank.

2000-2001 observations of glowing fumaroles and release of magmatic gas

Santa Ana's summit crater contains an acid lake that in 2000 was ~200 m in diameter with a maximum depth of 27 m; its volume was estimated at ~200,000 m3. The lake water has a composition typical of acid-sulfate-chloride lakes (table 1). Dissolved sulfates yielded delta34S of 16.0, suggesting that a significant sulfur source is magmatic SO2 gas. Several hot springs with 80°C temperatures lie along the shore of the lake and have compositions close to that of the lake waters. Fumaroles on the W side of the lake had a maximum temperature, recorded in January 2000, of 523°C.

Table 1. Compositions of Santa Ana's crater lake waters for 2000-2001. Samples were collected in 2000 during January (sample designation, SAL), July (SAN1), and August (SAN2), and collected in 2001 during February (SAP). Chemical concentrations are in mg/L. Courtesy of the authors.

The Centro de Investigaciones Geotecnicas (CIG) noted that during May-September 2000 the lake temperature increased from 19 to 30°C. On 15 February 2001, the temperature was 26°C and the pH was 0.8-0.9. Bubbling and increased gas emissions were observed in several areas of the lake. During February 2001, sulfur spherules in the lake caused the water color to change to a shade of milky yellow-brown. During May-September 2000, the composition of the lake was only slightly affected, suggesting that no major changes occurred within the hydrothermal system beneath the lake (table 1).

Beginning in August 2000, observers found the increasingly deleterious effects of acidic vapor and rainfall on vegetation in the area SW and N of the crater. Winds are dominated by NE trades, which generally drive the plume from the volcano over the rim of the crater to the S and W. By December 2000, more severe effects on the flora were reported, and an area of ~8-10 km2 located S and W of the crater contained markedly discolored and defoliated vegetation. Brief periods of acidic rainfall were reported at Juayua, 13 km W of the crater. In January 2001, incandescent areas within the fumarolic region W of the crater lake were observed at night and may have been present earlier.

COSPEC measurements of the plume were made using the Guatemalan COSPEC on 8 and 9 February 2001. Tripod-based surveys were made from Cerro Verde (elevation ~2,000 m), 2 km S of the crater. Automobile-based traverses conducted along the Santa Ana - Sonsonate highway (5 km W of Santa Ana) were made on 9 February. These surveys resulted in an average SO2 flux of 393 tons/day on 8 February and 244 tons/day on 9 February. During measurements of the plume from the Cerro Verde site, periods of noticeable puffing were observed, affecting the COSPEC measurements by a factor greater than 8.

Two new, permanent telemetered seismic stations, located 1.5 km SE of the crater and 5.5 km NW on Cerro Retiro, were installed by USGS/VDAP and CIG personnel in February 2001. During February, these stations did not detect abnormal seismicity beneath the volcano. A portable recorder located at Finca San Blas from about 20 January through 9 February registered little volcano-seismic activity apart from a few small fumarolic emissions in late January.

Interpretation: The changes since the summer of 2000 are apparently due to increased venting of a well-developed hydrothermal system through the lake, hot springs, and fumaroles. This hydrothermal system is venting substantially higher SO2 (and presumably other gases) in an acidic plume blown by NE trade winds. The lack of seismic activity suggests that the hydrothermal activity increase was not driven by the arrival of new magma beneath the crater.

The SO2 emission rate was high for a quiescent volcano that lacks an open vent. The SO2 may evolve from a gas reservoir below the hydrothermal system, trapped by a hydrothermal cap (clay or silica) as a result of long term (centuries or millennia) crystallization of magma below the cap (Giggenbach and others, 1990). The recent increase in degassing may reflect fracturing or leaking of the hydrothermal cap.

Crater hazards. Reports suggest that significant phreatic activity could occur at Santa Ana, including ejected bombs and blocks, even without magmatic movement under the volcano. This eventuality, a potential extension of the pulsating SO2 emissions, could make conditions unsafe in the crater region. Furthermore, unsubstantiated rumors from local residents and ambiguous geological observations (e.g., abundant dust-possibly settled ash-coating the upper surfaces of leaves of plants in the crater and on the upper slopes) have suggested to some that minor, rare phreatic eruptive events have already occurred.

January 2001 earthquake. At about 1135 on 13 January 2001, an earthquake with a magnitude of 7.6 and a depth of 60 km occurred off the El Salvadoran coastline, its epicenter (at 12.8° N latitude and 88.8°W longitude) lay ~150 km SE of Santa Ana volcano. The earthquake caused extensive damage and destruction throughout much of El Salvador. By 19 January the country was struck by over 660 aftershocks. According to information provided by the National Emergency Committee (COEN) on 17 January 2001, the death toll was put at 681, with 2,615 injured. Approximately 20,000 people moved into over 80 temporary shelters; 90,929 houses sustained damaged, with 24,759 destroyed; and 45,842 people had been evacuated.

News sources based on reports from local residents raised concern that the earthquake was the result of eruptions from Santa Ana volcano. This conclusion was almost certainly in error.

However, during a helicopter overflight on 17 January, James Vallance, an American volcanologist, believed he saw incandescence in the crater. During a subsequent overflight on 18 January, Vallance, Carlos Pullinger, and Demetrio Escobar observed that the incandescence came from glowing cracks in the fumarole field, which, as noted above, had measured temperatures exceeding 500°C. They noted that there was no new lava or magma visible in the crater to indicate recent eruptions. This conclusion was substantiated later by measurements with a portable seismograph, which failed to detect local earthquakes under the volcano.

This report discusses a 1 October 2005 eruption at Santa Ana (also called Ilamatepec) that sent a plume to 14 km altitude and led to initial estimates cited in the press of two deaths (perhaps from landslides), several injuries, and the evacuation of over 2,000 people. Observations of glowing fumaroles and release of magmatic gas during 2000-2001 were previously reported at Santa Ana (BGVN 26:04). Servicio Nacional de Estudios Territoriales (SNET) scientists noticed that between the summer of 2000 and April 2001 there was increased venting of a well-developed hydrothermal system through the crater lake, hot springs, and fumaroles, but these changes were not accompanied by detected seismicity, which was then taken to suggest that the increase in hydrothermal activity was not driven by the arrival of new magma beneath the crater. An ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) image from 3 February 2001 shows the volcano's setting well before the eruption (figure 1).

Figure 1. An ASTER image of Santa Ana from 2001 featured in one of several Earth Observatory reports. N is to the top left of the image and Santa Ana is the large, blunt-topped edifice closest to the left side of the image. In the color version of this image can be seen a tiny blue spot in the center of the inner-most crater?a crater lake (often called the lagoon). Behind Santa Ana is a large (7-km-diameter) lake inside the Coatepeque caldera. In the center is Izalco volcano, with dark-colored historical lava flows. Courtesy of NASA's Earth Observatory.

SNET reported that a sudden eruption at Santa Ana took place around 0820 on 1 October 2005. They estimated that it produced an ash-and-gas plume to a height of ~ 10 km above the volcano. According to the Washington VAAC, ash was visible on satellite imagery at an altitude of ~ 14 km. The US Air Force Weather agency provided images of the plume (figure 2).

Figure 2. Two images of a Santa Ana eruptive plume on 1 October 2005. (top) The plume at 1516 UTC; (bottom) the plume at 1650 UTC. Note that the label 'FL 460,' stands for 'flight level 460,' which is equivalent to an altitude of 46,000 feet or 14 km. Courtesy of the US Air Force Weather Agency.

Ash fell in towns W of the volcano, including in Naranjos, Nahuizalco, Juayúa, Ahuachapán (NW), and La Hachadura (at the border, ~ 40 km W, figure 3). SNET produced a graphic similar to an isopach map that showed near-source thicknesses provisionally to over 10 cm. The 10 cm isopach stretched ~ 5 km W; the 1 mm isopach, ~ 20 km W. The outermost isopach, presumably where measurable ash fell, was not closed; instead it was cut off along the Guatemalan border (~ 40 km to W of Santa Ana) and the caption said that ash would fall into valleys in Guatemala and to the sea. Volcanic blocks up to a meter in diameter fell as far as 2 km S of the volcano's crater. Lahar deposits were seen SE of the volcano. The alert level within a 4-km radius around the volcano's central crater was raised to Red, the highest level.

Figure 3. Graphic from SNET showing ashfall distribution from Santa Ana that appeared in the newspaper, La Prensa Grafica, following the 1 October eruption. N is upwards; Santa Ana lies ~ 40 km E of the Guatemalan border. This clearly transmitted the message that the ashfall was variable and W-directed over parts of El Salvador and neighboring Guatemala. The bottom of the graphic discussed the impact of the ash fall, including damage to specialty coffee farms. Credit: Ricardo Orellana, La Prensa Grafica.

According to news reports, two people were killed by landslides (possibly caused by heavy rain in the area) in the town of Palo Campana, and thousands of residents near the volcano were evacuated. As many as 1,400 hectares of crops were damaged by ash (1 hectare = 10,000 m2). News also mentioned other processes such as a flood of boiling mud and water, and molten rocks, some the size of small automobiles, that will be discussed in later reports. A several-minute-long video from the LPG Television website appears as both a hyperlink and an active file on our website. In addition to numerous interviews with evacuees, it shows a host of features including what appear to be the swaths left by previously inflated mudflows passed down steep-sided valleys.

Prior to the eruption, no significant change in seismicity was observed. On 3 October, after the eruption, seismicity fluctuated and small explosions occasionally occurred. Earthquakes associated with explosions were recorded. In addition, there was a decrease in the amount of SO2 emitted from the volcano.

Strong degassing had been measured at the volcano since June 2004. An ash emission occurred on 16 June 2005, and a slight increase in seismicity and a significant increase in gas emission were measured from 27 July until at least 30 August. SNET also reported a significant increase in seismic activity at Santa Ana on the night of 27 August. A cluster of 17 volcano-tectonic earthquakes were recorded, with four located S of the volcano. Afterwards, continuous high-frequency tremor was recorded until at least 30 August. Observations made on 29 August revealed incandescent rocks in the fumarole field, effects attributed to hot gases heating the rocks to sufficient temperature to glow. A significant increase in SO2 emission was recorded, and gas-and-steam plumes rose 500-1,000 m above the volcano's crater. As a safety measure, access to the volcano's crater was restricted.

From 27 July until the eruption on 1 October, seismicity and gas emissions were above normal levels, and Santa Ana was at alert level yellow. During the first week of September, tremor continued to be recorded, and on 2 September a cluster of at least eight small earthquakes occurred, which were not felt by local residents. Gas plumes rose to ~ 500 m above the volcano, and the SO2 flux was over 1,000 metric tons per day during the first two weeks of September. Satellite imagery from 5 September showed a thermal anomaly.

Microseismicity increased significantly on 12 September. During a visit to the volcano on 8 September, larger areas of incandescence were visible at a field of fumaroles than during a visit on 29 August. Satellite imagery showed a thermal anomaly at the volcano on several days during the second week of September.

During 15-19 September gas plumes rose to ~ 500 m above the volcano, and the SO2 flux reached a maximum of 3,320 metric tons per day on 16 September. Microseismicity remained at relatively high levels. No significant changes were seen at the volcano's crater when observed on 19 September in comparison to 13 September. Intense degassing continued and the crater lake (lagoon) remained a dark coffee color. Incandescence was visible inside some cracks.

During a visit to the crater on 21 September, observers noted that the lagoon had become greener and small rock landslides occurred in the field of fumaroles. Gas plumes rose to ~ 1 km above the volcano on 26 September.

Following the eruption of 1 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana during 5-11 October. Inclement weather prohibited ground and satellite observations, and sulfur-dioxide (SO2) measurements during much of the report period. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater. Around 11 October, SO2 measurements were around 600-700 metric tons per day. The alert level within a 5-km radius around the volcano's central crater remained at Red.

Previous comments regarding terminal phases of the 1 October 2005 eruption (BGVN 30:09) included: . "Following the eruption of 1 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana during 5-11 October . . .. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater."

Carlos Pullinger (Servicio Nacional de Estudios Territoriales, SNET) later noted that "The 1 October eruption only lasted about 1 hour. After that we had some small activity, probably associated [with] degassing on Sunday evening [2 October] and at about the same time the continuous rains produced the first of a series of lahars that affected the communities close to the shore of Coatepeque lake. During the rest of the week it was very difficult to know what was going on because of continuous rains and cloudy conditions."

Pullinger further noted that some eye witnesses said that they had observed a column on 2 October. SNET registered strong and continuous tremor during approximately 1900-2400 (local time) on 2 October. Much of this activity coincided with rain-induced lahars. Over 300 mm of rain fell on the volcano that day. Using both witness reports and seismicity, SNET inferred that on 2 October the volcano possibly generated strong degassing or even geyser-type activity. However, there was no confirmation of ashfall deposits from these or other post-1 October events.
The same type of seismicity continued intermittently until 5 October, but with much less intensity than on 2 October. SNET could not tell if there was any volcanic activity related to these events, or if it was mainly lahars. After the 5th continuous tremor was not recorded.

Figure 4. A graph showing Santa Ana's SO2 flux (vertical bars) and average daily seismic amplitude (RSAM, solid line) during 15 August-31 December 2005. The eruption of 1 October 2005 is indicated with an arrow. Courtesy of SNET.

Storms on 12 October 2005 caused lahars that traveled E towards Lake Coatepeque (see ASTER image of the region in BGVN 30:09). On 22 October, a lahar was reactivated in the Potrero Arriba area, NE of the volcano. During 22-25 October, the volcano was subjected to increased tremor and a slight increase in seismicity associated with gas emissions. On 28 October volcanic activity appeared to increase slightly and sulfur-dioxide emission rates during 28 and 29 October averaged 257 metric tons per day. The Alert Level within a 5-km radius around the volcano's central crater was at Red, the highest level.

During the month of November 2005 seismicity, volcanic activity, and gas emissions all remained for the most part at relatively low levels. There were slight increases on 13, 17, and 26 November; but the 17 November increase was attributed to noise from strong winds. On 26 November only slight changes were noted in the color of the lagoon in the crater's interior, but gas emissions rose to ~ 300 m above the volcano. Small earthquakes occurred during November 2005, inferred to be associated with the fracturing of rocks and gas pulses. Sulfur-dioxide emissions were low during the first part of November, with 100 to 200 metric tons recorded daily, and during the latter part of November, with between 100 and ~ 1,500 metric tons recorded daily.

During December 2005, seismicity was above background levels. Observations of Santa Ana's crater on 28 December revealed that there were continuous emissions of steam and gas from the lagoon and fumaroles located within the crater (figure 5). Gas rose 200-500 m above the crater and drifted SW (figure 6). Small earthquakes occurred, but gas emissions rose to over ~ 2,500 tons per day (figure 4). The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Figure 5. A photo taken from the crater rim at Santa Ana showing steam and gas emissions from both the lagoon and fumaroles located within the crater. Courtesy of SNET.

From 30 December 2005 to early January 2006, seismic and steam emissions were moderate at Santa Ana. Seismicity was slightly above normal levels with small earthquakes occurring, which were interpreted as being associated with gas pulses. Low-level emissions of steam and gas from the lagoon and fumaroles within the crater remained the same as in December 2005. Gas rose 200-500 m above the crater and drifted SW. The sulfur-dioxide flux ranged between 180 and 1,476 metric tons per day. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Background. Santa Ana, El Salvador's highest volcano, is a massive, 2,381-m-high andesitic-to-basaltic stratovolcano that rises immediately W of Coatepeque caldera. Collapse of the volcano during the late Pleistocene produced a voluminous debris avalanche that swept into the Pacific Ocean, forming the Acajutla Peninsula. Reconstruction of the volcano subsequently filled most of the collapse scarp. The broad summit of the volcano is cut by several crescentic craters, and a series of parasitic vents and cones have formed along a 20-km-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro la Olla cinder cones on the SE flank. Historical activity, largely consisting of small-to-moderate explosive eruptions from both summit and flank vents, has been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 13 km to the E.

Our last report (BGVN 31:01) discussed post-eruption lahars following the sudden 1 October 2005 eruption (BGVN 30:09). This report contains two sections. The first section addresses regional processes such as vegetation loss, ash accumulation, and lahars on and beyond the E flank of Santa Ana (also known as Ilamatepec) to the shores of Lake Coatepeque. Those lahars began soon after the 1 October 2005 eruption. The information on these lahars chiefly came from a report (SNET, 2006) authored by El Salvador's Servicio Nacional de Estudios Territoriales (SNET).

The second section addresses monitoring and observations such as extensive steaming and drop in the surface elevation of the lake in the summit crater. Material for this section, primarily found on the SNET website, covers January-April 2006, when activity was fumarolic with no large eruptions.
The 1 October 2005 eruption was possibly followed by a second one two days later on 3 October (SNET, 2006). A 3 October eruption was not mentioned in previous Bulletin reports.Carlos Pullinger explained that the evidence for the second eruption was tremor that day, but that could stemmed from other causes such as geysers in the summit crater lake, so the evidence for a 3 October eruption remains equivocal.

E-flank issues. October 2005 volcanism took place coincident with unusually high rains during tropical storm Stan (1-10 October 2005). On the E flank, the October 2005 eruptive episode killed extensive vegetation and left loose ash deposits covering the upper slopes (figure 7).

Figure 7. A November 2005 photo looking southward showing Santa Ana in the foreground, along with denuded, ash-laden vegetation. A wisp of steam escapes the summit crater, a basin hosting an acidic crater lake. Santa Ana's plumes and October 2005 ash deposits, coupled with other factors such as steep slopes, stress to vegetation, the lack of surviving permeable soils, and regional rainfall have led to a rash of new E-flank lahars. Peaks beyond Santa Ana include its satellitic cone Cerro Verde and then Izalco (sharp peak beyond the notch). Photo from SNET (2006).

Based on a rain gauge 5 km W of the crater (national meteorological station Los Naranjos), rainfall in October averages 193 mm; the yearly average is 2,155 mm. In the months prior to October 2006, rainfall at that station remained at normal values, always below 460 mm per month. In contrast, rainfall reached 865 mm during October 2006. During the peak of the storm, 3-6 October 2005, the Los Naranjos rain gauge collected more than 100 mm per day; the highest reading of 320 mm was on 5 October.

The lahars on Santa Ana's E slope consisted of both material from the October 2005 eruption as well as previous deposits. The first lahar seen by local witnesses took place on the night of 2 October 2005. It carried material up to 2 m in diameter. The lahars that produced most of the damage were those that occurred immediately after the eruption and reached a maximum thickness of 1.5 m. Other lahars descended later in the storm, persisting well into 2006.
The 2006 rainy season did not generate damaging lahars, just heavy runoff with minor sediment. In all, SNET seismically registered 22 lahar events, all of which were confirmed by local residents. The communities used tractors used to keep the main drainages open and to build levees, which confined the lahars inside main drainage areas. The SNET website mentioned several lahar episodes during 2006. Some of these episodes occurred in May, June, and July 2006.

A large scallop in the topographic margin of Coatepeque caldera results in Planes de la Laguna (an area of ~ 10 km2), which was where lahars eventually deposited (figures 8 and 9). This area of less steeply sloped, and in places comparatively level, ground contains numerous coffee plantations and small settlements. The largest settlement is El Javillal (figure 8, adjacent Lake Coatepeque).

Figure 8. Lahars displayed as trains of heavy dots on a topographic base map of the E-central side of Santa Ana and the adjacent W side of Lake Coatepeque. (N is towards the top; light grid-lines are 1 km apart, so the distance from the summit on the W to the large lake on the E is ~ 6.5 km.) In general, the lahars descended from W to E. Coatepeque is a 7 x 10 km caldera and the series of dashed lines across the map indicate the caldera's steep-sided topographic margin in. Several caldera domes are labeled, including Cerro Pacho and Cerro Afate. Note the lahar entering the settlement adjacent Lake Coatepeque ("Caserío El Javillal"). From SNET (2006).

Figure 9. An E-W topographic profile with Santa Ana on the W across to the E side of Lake Coatepeque on the E. Dashed lines indicate the location of Coatepeque's caldera wall. From SNET (2006).

The upslope areas contained numerous channels carrying lahars (figure 8). Several kilometers into the caldera the channels merge as they cross the less steeply sloped Planes de Laguna. The channels eventually grow into two primary channels, La Mina on the S and El Javillal on the N (figure 10). The La Mina channel led directly towards the Cerro Pacho dome, where the lahars proceeded to branch into multiple routes (A, B, C, and D) before entering El Javillal (figure 11).

Figure 10. Annotated aerial photo at unknown date showing part of Coatepeque's Planes de Laguna, W of Santa Ana, taken looking roughly S. The view illustrates lahars in and around El Javillal.The lahars entered the area along two drainages (Quebradas La Mina and El Javillal), both flowing from right to left (arrows). Adjacent to the domes and settlements, the flow patterns become quite complex (as indicated by flow directions A, B, C, and D). Lake Coatepeque appears at the upper left. The steep caldera wall lies along the photo's margin from the upper center to right corner. The large circular dome is Cerro Pacho; the smaller dome to the right is Cerro Guacamayero. Photo from SNET (2006).

Given the lack of soils and the state of vegetation, lahars were viewed as a potential ongoing hazard. To control lahars, SNET (2006) proposed excavating two channels from the vicinity of the domes to Lake Coatepeque, to carry sediment farther towards the lake. The proposed artificial channels are 2 m deep, with sides that slope at 45° outwards, and with a flat floor 5 m across. One proposed channel follows the S margin of the Cerro Pacho dome, the other follows a path similar to arrow A on figure 10.

Pullinger noted that the jocote de corona crop harvest was not affected because it came out just after the eruption. However, coffee was damaged wherever ash fell. Lahars did not directly hurt coffee plantations, but access roads were damaged and labor for harvesting was minimal, after much of the population had fled.

Monitoring. Moderate seismic activity and steam emissions continued during 2006. During 2006, seismicity was slightly above normal levels. Small earthquakes were interpreted as being associated with gas pulses.

Degassing continued in January 2006 with sporadic gas-and-steam emissions which rose approximately 200 m before dispersing. The SO2 flux ranged between 163 and 1,578 metric tons/day.

On 2 February, there was an increase in seismicity, possibly related to an earthquake on the coast of Guatemala. From 1-7 February the SO2 flux averaged 2,000 metric tons per day. A drop in the water level of the steaming, green-colored acidic lake in the summit crater revealed a local topographic high in the lake's center, which took the form of an irregular island (figure 12).

Figure 12. Photo showing the crater lake at Santa Ana volcano. The decrease in the water level has revealed an island of rocks and sediments that was previously covered by the crater lake. Photo taken on 17 February 2006 and provided courtesy of SNET.

Intense bubbling and fumarole activity during 27 February-23 March disturbed the lake's surface and made it difficult to assess the level of the water. During April, instability in the crater led to periodic landslides. One significant landslide deposited material in the SW section of the beach of the crater lake.

Researchers from Michigan Technological University (MTU) and Servicio Nacional de Estudios Territoriales (SNET) visited the crater of Santa Ana on 28 June and 5 July 2007 to measure crater lake and fumarole temperatures, and to carry out routine water sampling.

Crater lake. The crater lake appeared yellowish-green and had a maximum temperature of 57.5°C, measured by a thermocouple at the northern shore. The crater lake was observed to have shifted westward in position since the 1 October 2005 eruption, drowning the main pre-eruption fumarole field to the W and receding from its eastern border (figure 13). A subaqueous hot spring was observed in the center of the lake at the end of a peninsula of exposed sediments (figure 14). The hot spring exhibited episodic pulses of bubbling water about every 5 minutes.

Figure 13. The yellowish-green acid crater lake of Santa Ana volcano as seen when viewed on 28 June 2007 looking towards the N. Photo taken by Anna Colvin.

Figure 14. Hot spring emerging in the acid lake at Santa Ana as seen 5 July 2007. Episodic upwelling of whitish fluid radiated out from the base of the large rock in the center of the photo. View is towards the SW; note geologist for scale. Photo taken by Matt Patrick.

Fumaroles. Crater fumaroles were observed to the W and S of the crater lake, and weak fumaroles were also observed on the upper wall above the flat area and below the SW crater rim. The southern crater fumaroles and the upper fumaroles were measured by thermocouple and radiometer (Extech 42545) (figure 15). Fumaroles to the W were not measured due to limited accessibility.

Figure 15. At Santa Ana, the location of fumarole measurements and the hot spring shown in the previous figure. View is towards the SW. Photo mosaic taken 5 July 2007 by Matt Patrick.

The seven largest southern crater fumaroles were measured along an E-W transect. The lower fumaroles emitted mainly water vapor, though some sulfur crystals and a weak sulfurous smell were present. Lower fumaroles temperatures ranged from 92.0 to 95.2°C, and thermocouple and radiometer measurements agreed very well (to within 3%). The upper fumaroles were diffuse and relatively weak, occurring in loosely consolidated tephra. The upper fumaroles emitted mainly water vapor and lacked sulfur deposits or sulfurous smell. Upper fumaroles temperatures ranged from 70.0 to 79.0°C, and thermocouple and radiometer measurements agreed well (to within 6%).

Volcano Types

Rock Types

Tectonic Setting

Subduction zoneContinental crust (> 25 km)

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

489
21,653
1,240,131
6,486,880

Geological Summary

Santa Ana, El Salvador's highest volcano, is a massive, 2381-m-high, dominantly andesitic-to-trachyandesitic stratovolcano that rises immediately west of Coatepeque caldera. Collapse of Santa Ana (also known as Ilamatepec) during the late Pleistocene produced a voluminous debris avalanche that swept into the Pacific Ocean, forming the Acajutla Peninsula. Reconstruction of the volcano subsequently filled most of the collapse scarp. The broad summit of the volcano is cut by several crescentic craters, and a series of parasitic vents and cones have formed along a 20-km-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro la Olla cinder cones on the SE flank. Historical activity, largely consisting of small-to-moderate explosive eruptions from both summit and flank vents, has been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 13 km to the east.

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Ilamatepec

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Astillero, Cerro el

Pyroclastic cone

1480 m

13° 48' 50" N

89° 36' 22" W

Conejal, Cerro el

Pyroclastic cone

1495 m

13° 48' 32" N

89° 36' 40" W

Cruces, Cerro las

Pyroclastic cone

1140 m

13° 55' 0" N

89° 40' 0" W

Duraznillo, Cerro

Stratovolcano

13° 53' 0" N

89° 40' 0" W

Malacara
Ayeco

Stratovolcano

1908 m

13° 53' 0" N

89° 40' 0" W

Olla, Cerro la
Chino, Cerro

Pyroclastic cone

1323 m

13° 48' 14" N

89° 35' 38" W

Retiro, Cerro el

Pyroclastic cone

1460 m

13° 54' 0" N

89° 39' 0" W

San Marcelino
Teixcal

Pyroclastic cone

1240 m

13° 48' 32" N

89° 35' 0" W

Tamages
Tamagastepec

Cone

13° 53' 0" N

89° 42' 0" W

Verde, Cerro

Pyroclastic cone

2030 m

13° 50' 0" N

89° 37' 0" W

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Cuscachapa, Laguna de

Pit crater

710 m

13° 59' 0" N

89° 40' 0" W

Perol, El

Crater

13° 54' 0" N

89° 39' 0" W

Plan del Hoyo

Crater

1800 m

13° 52' 0" N

89° 38' 0" W

Pozo, El

Pit crater

900 m

13° 57' 0" N

89° 40' 0" W

Seca, Laguna

Pit crater

730 m

13° 59' 0" N

89° 40' 0" W

Photo Gallery

Santa Ana, El Salvador's highest volcano, is a massive, 2381-m-high stratovolcano whose summit is truncated by a series of four nested craters, seen here from the SW. A series of parasitic vents and cones have formed along a 20-km-long fissure system that extends from the low NNE flank to the San Marcelino and Cerro Chino cinder cones on the SE flank. Historical eruptions, largely consisting of small-to-moderate explosions from both summit and flank vents, have been recorded since the 16th century.

Photo by Mike Carr, 1982 (Rutgers University).

The broad flat-topped summit of Santa Ana volcano is truncated by a 1.5-km-wide crater seen here from the south. A series of bedded phreatomagmatic tephra layers exposed in the crater wall in this photo mantle the summit region and overlie lava flows exposed lower in the crater walls.

Copyrighted photo by Dick Stoiber, 1966 (Dartmouth College).

The steep-walled inner crater of Santa Ana volcano is half a kilometer wide and is partially filled by a 250-m-wide greenish crater lake. Fumaroles are located on the crater wall and in the lake, and abundant sulfur deposits are located on the SW wall (upper left). The acidic crater lake has a pH of about 1, and recent bathymetric surveys revealed that the lake was shallow, with a maximum depth of 27 m. Lake temperature in May 2000 increased to 30 degrees Centigrade.

Copyrighted photo by Dick Stoiber, 1966 (Dartmouth College).

The active crater of Santa Ana volcano lies at the SE end of series of four nested summit craters and contains a small lake not visible in this photo. The northern crater wall in the background exposes a series of lava flows and inter-bedded fragmental deposits. Several faults, such as the one at the upper left, are visible in the crater walls. The eastern side of Santa Ana's crater was breached and then refilled. Later tectonic movements produced a graben in the central part of the crater with displacements of 50-70 m.

Photo by Bill Rose, 1966 (Michigan Technological University).

Thermal activity at the surface of a volcano is evidence of volcanic heat below. The fumarolic activity seen here produces vigorous steam plumes along the sulfur-encrusted wall of the summit crater at El Salvador's Santa Ana volcano. Thermal activity is common during non-eruptive periods at many volcanoes and may persist for many thousands of years. In addition to the fumarolic activity in this photo, the interaction of high-temperature volcanic fluids and gases with groundwater in hydrothermal fields can produce geysers, hot-spring pools, and mudpots.

Photo by Kristal Dorion, 1994 (U.S. Geological Survey).

Cerro Verde, a 2030-m-high satellitic cone of Santa Ana volcano, is seen here from the NW above Hacienda San Blas. Cerro Verde is near the SE end of a 20-km-long eruptive fissure that cuts across Santa Ana from its lower NNE flank. Three N-S-trending craters are located at the summit of Cerro Verde, and a fourth is located on the SE flank of the cone.

Photo by Kristal Dorion, 1994 (U.S. Geological Survey).

Santa Ana's active crater, partially filled by an acidic crater lake, lies at the SE end of a series of four nested craters. This photo is taken from the northern rim of the next-to-youngest crater and shows a broad bench whose surface is dotted by several small phreatomagmatic vents formed during historical eruptions.

Scoria from the 1904 eruption of Santa Ana volcano form the darker deposits blanketing the rim of the summit crater. The first of two 20th-century eruptions from Santa Ana began on January 12 and lasted for about two weeks, during which phreatomagmatic explosions ejected these scoriae.

Two dramatic volcanoes rise above the town of Juayua (right-center) in western El Salvador. The conical peak at the left is 1961-m-high Cerro los Naranjos, one of the youngest peaks of the Apaneca Range volcanic complex. The broader peak at the right is 2365-m-high Santa Ana, El Salvador's highest volcano. Los Naranjos has not erupted in historical time, but Santa Ana has had eruptions from both summit and flank vents since the beginning of the Spanish era.

The broad summit of Santa Ana volcano is seen here from Cerro Verde, a satellitic cone on its SSE flank. A series of four nested craters, the largest of which is 1.5 km wide, truncates the summit. Houses of Hacienda San Blas are visible at the bottom of the photo. Much of the slopes of Santa Ana are covered with coffee plantations that are an important part of the local economy.

These bedded scoria layers from Santa Ana were transported intact with only slight disruption about 30 km from the volcano in the Acajutla debris avalanche. A more than 20-m-thick sequence of inter-bedded tephra layers and thin lava flows is exposed in this quarry in an avalanche hummock near Highway 2. The red-and-yellow bars on the scale mark 10-cm increments.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

The broad light-colored hill in the center of the photo is the Cerro la Olla-Cerro Marcelino cone complex, a flank vent of Santa Ana volcano, whose cloud-topped summit is seen at the right. Cerro Marcelino at the right side of the cone complex was formed during the 1722 eruption, when the Teixcal lava flow traveled 13 km to the east and overran the town of San Juan Tecpan. The rounded forest-covered peak at the left is Cerro Verde, a large pyroclastic cone on the SSE flank of Santa Ana.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

The flat-topped summit of massive Santa Ana volcano appears at the right, with conical Izalco volcano at the left in this NW-looking view from the Zapotitán basin. The pyroclastic cone on the center skyline is Cerro Verde, and the brownish scoria cone below it is Cerro Marcelino. Historical eruptions have occurred from the summit crater of Santa Ana and satellitic cones such as Cerro Marcelino on its SE flank. Izalco volcano was born in 1770, and the saddle between Izalco and Cerro Verde increased 100 m in height in the century after 1866.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Bedded scoria layers are exposed in a quarry on the NE flank of Cerro Verde volcano along the road to its summit. Cerro Verde is the largest of a chain of cinder cones on the SE flank of Santa Ana volcano. Cerro Verde has produced basaltic-to-andesitic products.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

Santa Ana volcano is seen here from the east on the rim of Coatepeque caldera. The eastern rim of Santa Ana's summit crater was breached and produced the gently sloping deposits extending towards the caldera lake at the lower right. The NW wall of the caldera in the background cuts into the flanks of Santa Ana volcano to a level about 800 m above the lake surface (lower right). This SW part of Coatepeque caldera was formed about 57,000 years ago during the eruption of about 16 cu km of rhyolitic pumice-fall and pyroclastic-flow deposits.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Izalco volcano was once known as the "Lighthouse of the Pacific" for its persistent incandescent nighttime displays. It was constructed on the southern flank of Santa Ana volcano, whose flat-topped summit is visible to the left of Izalco. At the right is Cerro Verde, a basaltic-to-andesitic pyroclastic cone on the SE flank of Santa Ana. Frequent strombolian eruptions have left the flanks of Izalco unvegetated; dark-colored lava flows at its base issued from both summit and flank vents and extend up to 7 km from the volcano.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Izalco has been El Salvador's most active volcano during historical time. More than 50 eruptions, many of several years to about a decade duration, took place since 1770, when the volcano was born. During its 200 year history prior to 1966, when activity ceased, the volcano was rarely quiet for more than a few years. Eruptions took place both from the summit craters and from flank vents. Unvegetated lava flows are seen here on the SE flank below Cerro Verde volcano (center) and the twin cones of El Conejal and El Astillero (right).

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

The broad E-W-trending Apaneca Range is seen here from the south with flat-topped Cerro de Apaneca on the left. The Las Ninfas-Laguna Verde complex is in the center, and to its right are Cuyanausul, Cerro de la Rana, Cerro Aguila, and Cerro los Naranjos. The 5-km-wide Concepción de Ataco caldera lies beyond the center horizon, its rim obscured by post-caldera cones. The photo is taken from the top of a hummock on the surface of the massive Acajutla debris-avalanche deposit, which originated from Santa Ana volcano, out of view to the right.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Santa Ana volcano, seen in the distance more than 30 km to the north, underwent catastrophic edifice collapse during the late Pleistocene. This produced a massive, highly mobile debris avalanche that deposited the boulders in the foreground and formed the small hills in the middle distance. The larger hill at the left is a kipuka formed by rocks of the older Bálsamo formation, which were surrounded by the avalanche. This avalanche was one of the largest known in Central America and traveled nearly 50 km from the volcano.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Santa Ana volcano rises beyond the summit crater of Izalco volcano, which formed beginning in 1770 on the northern flank of Santa Ana. Fumarolic activity producing the faint steam plume in the right foreground continues at Izalco, but has diminished considerably since the cessation of its 200-year-long eruptive period in 1966. A complex of four nested craters gives the summit of Santa Ana a flat profile. The slopes of Cerro Verde, a satellitic cone of Santa Ana, are seen at the right.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

The blocky basaltic-andesite lava flow in the foreground was erupted in 1722 from San Marcelino, the light-colored cinder cone on the right horizon. The 13-km-long Teixcal lava flow traveled to the east and destroyed the small village of San Juan Tecpan. The flow originated from two vents on opposite sides of San Marcelino. The peak on the center horizon is Cerro Chino, the central of three cinder cones along a line extending SW from San Marcelino to La Olla.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

Large angular clasts in an oxidized scoria matrix of the Acajutla debris-avalanche deposit are exposed at the Pacific Ocean coastline more than 40 km from Santa Ana volcano. Clasts with a jigsaw fracture pattern can be seen at the top of the photo. Large clasts to 6 m in exposed dimension occur nearby. Note the rock hammer in the center for scale.

Photo by Lee Siebert, 1999 (Smithsonian Institution).

An aerial view of the western side of Coatepeque caldera shows Cerro la Isla lava dome forming the island at the left center. On the ridge behind the caldera wall are (from left to right) the San Marcelino-La Olla and El Conejal-El Astillero pyroclastic-cone complexes, the tip of Izalco volcano, rounded Cerro Verde scoria cone, and (in the clouds) the summit of Santa Ana volcano. The Pacific Ocean is visible in the distance.

Photo by Bill Rose, 1967 (Michigan Technological University).

The escarpment cutting diagonally downward across the middle of the photo, its face highlighted by vertical rows of trees in coffee plantations, is the NW wall of a large caldera formed by edifice collapse of Santa Ana volcano during the late Pleistocene. About 5 km of the avalanche caldera rim is exposed; the remainder is buried beneath ejecta and lava flows from modern Santa Ana volcano. Conical Cerro los Naranjos volcano rises beyond the scarp, and other peaks of the Apaneca range form the horizon on either side.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

San Marcelino cinder cone (center) on the NE flank of Santa Ana volcano was the source of a basaltic-andesite lava flow that traveled 13 km to the east (left). Larde (1923) considered the 1722 lava flow to have originated from Izalco volcano, but most other sources place the birth of Izalco in 1770 and assign the 1722 eruption to San Marcelino. The 1722 lava flow originated from two vents at the eastern and western sides of San Marcelino.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

The massive Acajutla debris avalanche swept into the sea over a broad front, forming a peninsula that extended the shoreline about 7 km. The rocks in the foreground and across the bay at Playa Dos Cobanos mark the farthest subaerial extent of the avalanche, 41 km from its source at Santa Ana volcano, whose cloud-capped summit is on the right horizon. Hummocks are exposed to the coastline and are visible well offshore on bathymetric surveys, which suggest that the avalanche had a significant submarine component.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Grass-covered San Marcelino cinder cone is seen from Cerro Chino on the SE flank of Santa Ana volcano. San Marcelino was formed during an eruption in 1722 and produced a lava flow that traveled 13 km to the east. The forested southern rim of Coatepeque caldera can be seen to the NE beyond and to the left of San Marcelino, and a small sliver of the surface of Lake Coatepeque is visible inside the caldera.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

The brownish area extending across the center of the photo is the Teixcal lava flow. Originating during an eruption in 1722 from the base of San Marcelino cinder cone (out of view to the right) on the SE flank of Santa Ana volcano, the flow traveled 13 km to the east to the edge of the Zapotitán basin and destroyed the village of San Juan Tecpán, burying about 15 sq km of prime agricultural land. Numerous elongated trenches on the surface of the blocky basaltic lava flow may mark the collapsed roofs of lava tubes.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

The brownish area extending diagonally from the lower left towards San Salvador volcano on the right horizon is the Teixcal lava flow. It originated during an eruption in 1722 from the base of San Marcelino cinder cone, from where this photo was taken. The basaltic-andesite flow is noted for its disequilibrium textures, where large ortho-pyroxene crystals are surrounded by reaction rims of olivine. The half-forested, half-grassy cone at the left center is Cerro Alto, a flank cinder cone of Coatepeque caldera, out of view to the left.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

A 20-km-long NW-SE-trending chain of satellitic vents cuts across Santa Ana volcano. This view looks to the NW from Cerro Chino cinder cone at the SE end of the chain. The small forested cone at the middle left is Cerro el Astillero, and the larger cone on the left horizon is Cerro Verde. The broad summit of Santa Ana forms the right horizon.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Several nested craters are found at the summit of Santa Ana volcano. Steam rises from fumaroles on the steep-sided NE wall of an inner crater, which cuts the flat-bottomed floor of a roughly 900-m-wide crater. The 80-m-high walls of this crater expose bedded tephra layers from phreatomagmatic eruptions. The walls of the two outer craters, which are breached to the SW, appear at the upper right.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Thick sequences of bedded phreatomagmatic tephra layers more than 100 m thick are exposed in the southern and western crater walls of Santa Ana volcano. The hydrothermally altered area at the lower right is the inner crater, which contains an acidic lake not visible in this photo. Four crater walls can be seen in this photo; the two outer walls appear on and below the center horizon, beyond the flat line that marks the rim of the 2nd crater.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

The rugged eastern crater rim of Santa Ana volcano rises more than 100 m above the crater floor. The eastern wall is composed of breccias, scoria layers, thin lava flows, and dikes. Thick sequences of lava flows are exposed in the northern and southern crater walls and are overlain by bedded phreatomagmatic tephra layers up to 100 m thick on the southern side.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Three peaks of the Santa Ana massif can be seen from the highway between Sonsonate and San Salvador. The summit of Santa Ana volcano, barren as a result of historical eruptions and recent gas emission, is at the left, and conical, unvegetated Izalco volcano is at the right. Forested Cerro Verde, one of a series of cones formed along a SE-NW line cutting across Santa Ana, is the rounded peak on the right-center horizon.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Santa Ana volcano rises above scenic Lago de Coatepeque. The 6-km-wide lake lies at the eastern end of the compound Coatepeque caldera. The northern and southern rims of the western side of the caldera are visible beyond the lake. The rounded peak to the left of Santa Ana is Cerro Verde, one of many cones constructed along a NW-SE-trending fissure cutting across the Santa Ana complex.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Coatepeque caldera is seen here from its northern rim, with massive Santa Ana volcano in the background. Light-colored areas at the left and center are part of the caldera walls, which rise from 250 m to about one kilometer above the lake. The rounded summit behind the caldera rim at the left is Cerro Verde, which was erupted along a NW-SE-trending fissure cutting through Santa Ana.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

Multi-colored remnants of Santa Ana volcano are exposed in a quarry wall in a hummock of the Acajutla debris-avalanche deposit. This quarry in Cerro el Jicaro, 6 km SE of the city of Sonsonate, displays characteristic textures of debris-avalanche deposits. Individual segments are faulted and slightly deformed, but retain their integrity despite being transported about 18 km from the volcano. The geologist at the lower right provides scale.

Photo by Paul Kimberly, 1999 (Smithsonian Institution).

Fumarolic plumes are visible on the hydrothermally altered western wall of Santa Ana's inner summit crater, which contains an acidic lake. A steep descent down the NE crater wall at the far right allows access to the lake for geochemical sampling; recent surveys measured pH values of around 1. Sequences of bedded phreatomagmatic tephra layers more than 100 m thick are exposed in the background in the walls of the outer summit craters of Santa Ana volcano.

Photo by Paul Kimberly, 2002 (Smithsonian Institution).

Geologists on the southern rim of Santa Ana's summit crater observe the fumarolically altered walls of the more than 100-m-deep inner crater. A near-vertical fault left of center cuts the far northern wall of the larger crater beyond the broad bench in the center of the photo above the inner crater. The fault separates bedded phreatomagmatic tephra deposits on the left from a thick sequence of lava flows on the right. The lava flows are capped by a roughly 10-m-thick light-brown sequence of phreatomagmatic tephra layers.

Photo by Paul Kimberly, 2002 (Smithsonian Institution).

Multi-colored individual units of the Acajutla debris-avalanche deposit are offset along normal faults in this view of a quarry wall 6 km SE of the city of Sonsonate. Many blocks are unfractured, but intensely shattered blocks with jigsaw textures are also present. About 8 m of the quarry wall is exposed at the right-hand side. The voluminous Acajutla avalanche deposit was produced by late-Pleistocene edifice failure of Santa Ana; this quarry lies about 18 km from the volcano.

Photo by Lee Siebert, 2002 (Smithsonian Institution).

The escarpment cutting diagonally downward across the middle of the photo in front of conical Cerro los Naranjos volcano is the NW wall of a large late-Pleistocene caldera formed by edifice collapse of Santa Ana volcano. The exposed portion of the collapse scarp is up to about 200 m high along a roughly 5 km segment of the arcuate avalanche caldera. Pyroclastic ejecta and lava flows from modern Santa Ana volcano have filled in much of the scarp in the foreground and have buried it completely on the northern and eastern sides.

Photo by Paul Kimberly, 2002 (Smithsonian Institution).

The Acajutla Peninsula is the largest topographic irregularity along a 900-km-long stretch of the Pacific coast between the Gulf of Tehuantepec off Oaxaca, México and the Gulf of Fonseca at the SE tip of El Salvador. The 20-km-wide peninsula was created by a debris avalanche that traveled nearly 50 km following the late-Pleistocene collapse of Santa Ana volcano, located beneath the cloud bank to the right of Coatepeque lake. Part of the shallow submarine component of the deposit is visible in this Space Shuttle image with north to the lower left.

Lake-filled Coatepeque caldera is prominent in this Space Shuttle image with north to the lower left. A post-caldera lava dome forms an island at the SW side of the lake. The nested summit craters of Santa Ana volcano are visible below and to the right of Coatepeque, and the unvegetated cone of Izalco volcano is to the right of Santa Ana. Small stratovolcanoes of the Sierra de Apaneca form the forested ridge at the bottom right. The light-colored area at the left-center is the city of Santa Ana, the second largest in El Salvador.

Four volcanoes in western El Salvador are visible in this Space Shuttle image. The forested ridge at the left-center is the Apaneca Range, a complex of calderas and small stratovolcanoes. The summit crater of Santa Ana volcano lies below the small cloud bank, and the brownish area below it is Izalco volcano. A circular lake partially fills Coatepeque caldera. The Acajutla Peninsula at the bottom, named after the port city of Acajutla, was formed by a massive debris avalanche produced by the late-Pleistocene collapse of Santa Ana volcano.

This false color NASA aerial oblique ASTER image shows Santa Ana volcano (middle left), Izalco volcano (center), and lake-filled Coatepeque caldera from the SW. The summit of Santa Ana is truncated by a series of nested craters, and a NW-SE-trending fissure cuts across the massif. Fresh lava flows drape Izalco volcano, active from 1770 to 1966, and descend its southern flanks. The grayish area at the far upper left is the city of Santa Ana, El Salvador's second largest city.

The summit crater complex of Santa Ana volcano with its small light-bluish crater lake is visible at the left-center in this false color NASA ASTER image (with north to the top). The dramatic lake-filled Coatepeque caldera cuts the eastern side of the Santa Ana massif, and Izalco volcano and its historical lava flows lie south of Santa Ana. A NW-SE-trending fissure cutting across the massif was the source of an eruption in 1722 CE from a cinder cone (center) on the SE flank that fed the lava flow seen extending across the image to the lower right.

The summit crater complex of Santa Ana volcano with its small crater lake is visible left of center in this NASA ASTER image (with north to the top) taken on October 10, 2005, following an explosive eruption on October 1. In this false-color image dark ashfall deposits cover otherwise reddish vegetation around the summit of the volcano and extend down its upper eastern flank. The eruption occurred just prior to heavy rainfall accompanying Hurricane Stan, and lahars swept the flanks of the volcano. The large lake at the right is Coatepeque caldera.

An eruption plume from Santa Ana volcano towers above Coatepeque caldera lake. The brief, one-hour-long explosive eruption the morning of October 1, 2005 produced gas-and-ash plumes that rose 10 km or more. Ash fell in towns west of the volcano and extended into Guatemala; ashfall caused damage to coffee plantations. Volcanic blocks up to a meter in diameter fell as far as 2 km south of the summit. Lahars descended valleys on the flanks of the volcano.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).

Using infrared satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data, scientists at the Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, developed an automated system called MODVOLC to map thermal hot-spots in near real time. For each MODIS image, the algorithm automatically scans each 1 km pixel within it to check for high-temperature hot-spots. When one is found the date, time, location, and intensity are recorded. MODIS looks at every square km of the Earth every 48 hours, once during the day and once during the night, and the presence of two MODIS sensors in space allows at least four hot-spot observations every two days. Each day updated global maps are compiled to display the locations of all hot spots detected in the previous 24 hours. There is a drop-down list with volcano names which allow users to 'zoom-in' and examine the distribution of hot-spots at a variety of spatial scales.

Middle InfraRed Observation of Volcanic Activity (MIROVA) is a near real time volcanic hot-spot detection system based on the analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) data. In particular, MIROVA uses the Middle InfraRed Radiation (MIR), measured over target volcanoes, in order to detect, locate and measure the heat radiation sourced from volcanic activity.